The invention relates to a one-piece, steel welded cooling-channel piston of forged steel.
It is desirable to develop a piston surpassing the prior art in which the cooling channel or cooling cavity is tightly sealed. A piston of this type is described, for example, in the unpublished German patent application DE 10 2004 038 465.7.
A cooling channel piston is known from the previously published German patent application DE 102 44 512 A1 having the features which constitute the genre. The upper part of the piston has circumferential radial webs running coaxially behind the ring belt which are joined to corresponding webs on the piston skirt by means of friction welding. After the joining of piston upper part and piston lower part, the lower face of the ring belt is located adjacent the upper circumferential radial face of the piston skirt. In the state when piston upper and lower parts are joined, the outer piston wall areas between both parts of the piston form a contact surface which is characterized by a gap a few tenths of a millimeter wide. It can remain open or be sealed by means of a temperature-resistant sealing ring which is positioned before the joining of both parts of the piston on one of the contact surfaces, for example that of the lower part of the piston. A sealing ring of this kind has the advantage that the cooling channel is then closed to form a seal, but in a disadvantageous manner it represents an additional component which has to be produced and correctly positioned when the two parts of the piston are joined. In addition, the sealing ring, just like the few tenths of a millimeter wide gap without a sealing ring, has the disadvantage that the upper part of the piston (piston crown) cannot be supported on the lower part of the piston (piston skirt) while the piston is operating. While operating, the piston crown is deformed in a disadvantageous manner by the combustion pressures acting on it so that the strength and durability of the piston are compromised.
A cooling channel piston for a combustion engine is known from EP-A-1 061 249, having a piston crown and a piston skirt joined by means of friction welding, which together form a cooling channel, where a ring wall delimiting the cooling channel radially outward is formed by the piston crown and/or the piston skirt and the ring wall can be closed to form a seal by an interference fit and/or positive fit after piston crown and piston skirt have been joined. As shown in FIGS. 2 to 5, the surfaces of the ring belt and of the piston skirt facing each other are radially circumferential and coplanar with each other. The mutually coplanar facing surfaces on the lower edge of the ring belt and the upper edge of the piston skirt do not have any type of shoulder so that when the cooling channel piston is operating, there is no adequate support for the piston crown on the piston skirt. Only in FIG. 6 is it shown that the upwardly facing termination of the piston skirt has a single shoulder with which the coplanar aligned lower part of the ring belt can come into contact. This certainly improves the supporting effect, but not optimally. In addition, the inwardly directed L-shape of the shoulder at the upper end of the piston skirt is extremely difficult and expensive to manufacture with the required precision, so that no practical solution can be derived from EP-A-1 061 249.
Generic cooling channel pistons are also known from US 2004/144247 A1 and U.S. Pat. No. 6,698,391 in which, however, the circumferential radial lower edge of the ring belt and the upper edge of the piston skirt coming into contact therewith are also only configured coplanar so that the supporting effect is not disclosed.
It is therefore desirable to prepare a cooling channel piston which effectively avoids the disadvantages described above.